Fibrous sealing composition for porous formations and method for making the same



Unite fitates Homer L. Twining, San Antonio, Tex.

No Drawing. Application December 15, 1953, Serial No. 398,423

5 Claims. (Cl. 252-85) This invention comprises novel and usefulimprovements in a fibrous sealing composition for porous formations andmethod for producing the same, pertains generally to the art of drillingwells for the purpose of obraining oil, gas or other fluids fromsubterranean deposits, and more particularly has reference to a fibroussealing composition for cavernous or porous formations for preventingthe partial or complete loss of circulation during rotary drillingoperations or loss of the drilling fluid when drilling with cable toolsand to a method of manufacturing such a composition.

The present invention is similar in subject matter to that set forth inthe prior patent of John G. Campbell and Homer Twining, No. 2,599,745,granted on June 10, 1952 and constitutes improvements in suchcompositions and in the method of manufacturing the same; and is acontinuation-impart of my prior copending application, Serial No.267,700, filed January 22, 1952 for Drilling Fluid Additive, nowabandoned.

As employed in the specification, the term drilling fluid is understoodto cover all types of drilling fluids, particularly circulating liquiddrilling fluids and especially includes the numerous types of drillingmuds.

It is well understood that maintenance of circulation of the drillingfluid is vital to the operation of drilling and/or treating wells. Thefailure to maintain circulation, arising from the theft of the drillingfluid by the penetration into a porous or cavernous thief formation isknown as lost circulation or loss of returns and produces a considerablenumber of detrimental and often disastrous results which are so wellknown to those skilled in this 'art as to not require specificenumeration here. It is therefore the vital problem of the driller tomaintain circulation; or to re-establish and regain a lost circulation.

The problem of lost circulation is generally encountered in oil welldrilling by the rotary method. As the drilling fluid is circulated inthe bore hole under pressures exceeding that of the formation, the solidparticles suspended in the fluid and also the colloidal ingredients ofthe fluid flow into the formation through interstices, passages,fissures and pores of the formation material. Loss of circulation,however, is not encountered unless the; size of the interstices or poresincreases to the point where the clay particles, present as a componentof the drilling mud, cannot form a bridge across and thus seal theseopenings. The critical value of this pore diameter of a formation varieswith diflerent drilling muds, and even with the same mud, consideringthe fact that during the drilling process a portion of the clayformation penetrated usually becomes part of the drilling fluid, therebyaltering its characteristics. If the pore diameter of a zone exceedsthis critical value, then loss of fluid occurs until suspended solidparticles plug or reduce the pore size below the critical diameter,permitting the clay particles to function in forming a fluid impervioussheath. If the effective size of such pores can be sufficiently reducedas by bridging the same, the component clay parice ticles of thedrilling mud will be enabled to gather or pack into a non-porous massthereby sealing the porous formation and preventing further loss ofdrilling mud thereto.

It is well known that drilling fluids are of many different compositionsand require many varied characteristics in order to properly function inthe widely varied conditions and requirements of dilferent we'llformations. The compositions and properties of any particular drillingfluid are therefore critical for a particular well and must bemaintained within relatively narrow limits. Any additive admixedtherewith, as a lost circulation material or sealer, must be such as tonot detrimentally affect the characteristics of the mud as found to bedesirable for that well.

voluminous records carried by the well drilling industry conclusivelyestablish that the trend each year is toward deeper wells in the effortto find more oil. This trend results in steadily increasing hydrostaticpressures upon the drilling fluids, resulting in more frequent andserious loss of returns. In order to seal such losses, any sealing agentmust be capable of withstanding such relatively enormous pressures inorder to re-establish and maintain circulation.

A fibrous material, to be acceptable as a lost circulation additive,must possess the properties of high tensile strength, great flexibility,being chemically neutral and of not deteriorating after long periods ina drilling fluid.

In addition, it is extremely desirable that the fibrous material shallbe economical as to cost, capable of use in all of the varioustechniques and practices of well drilling, and capable of being readilyremoved from an oil bearing formation when completing a well.

It is believed that all of the known textile fibers possess theessential properties to some extent. As set forth hereinafter, rags andtextile fiber wastes are a satisfactory source for such fibers. Woodfibers, when properly prepared and conditioned, constitute asatisfactory and a more economical source of fibers than rags or textilefiber wastes.

The essential and fundamental purpose of this invention is to seal athief formation or prevent loss of circulation thereto.

Accordingly, an object of the invention is to provide suitable fibrouslost circulation materials or additives for mixing with drilling fluidsfor the purpose of preventing loss of circulation thereof and/ or torestore a lost circulation, which materials will be relatively simpleand inexpensive, will not detrimentally alter the characteristics of thedrilling fluids, and which will have such coaction with the voids andcavities of any permeable formation encountered in the drillingoperation as to reduce the effective pore size and enable theestablishment of a strong and reliable seal therefor.

Another object is to provide a drilling fluid additive which may consistsubstantially entirely of fibrous material or may comprise a mixture offibrous material and granular material in suitable proportions.

Another object is to provide a drilling fluid additive which may be usedwith other additives without detrimental effects to condition the fluidor prevent loss of the same.

Yet another very important purpose is to provide a drilling fluidadditive which shall be capable easily and with certainty to effectivelyseal thief formations under the increasingly heavy hydrostatic heads ofdeep well drilling.

In order to perform acceptably under present day requirements, asatisfactory lost circulation material must be amenable to variousmethods of application to a thief formation, as by the pill methodwherein a dose or slug of material is placed or spotted at a lostcirculation Zone with precision and economy and for maximum cl.-fectiveness.

A further object of the invention is to provide a drilling fluidadditive for .sealing porousformations-penetrated by the well bore andwhich will not interfere with the pumping of the drilling fluid, whichwill not deteriorate and which will not readily separate out of thedrilling fluid, even for long periods after its admixture with thefluid.

In the search of the industry for an inexpensive and all-purposedrilling fluid additive, many materials have been tried out, but only arelatively 'fewselected compositions are in use today, generally as .amixture of some fibrous material with granular material.

The following are some of the better known and more frequently usedadditives for improving the formation sealing rpropert-ies .of ordinarydrilling fluids: glue, casein, gelatin, gum arabic, cobal seaweed,cotton seed hulls, linseed cake, wheat flour, psyllium seed, micaflakes, asbestos fibers, shredded bagasse, shredded redwood bark, Balsamwood wool and paper pulp. Since the drilling well operator rarely has ameans of knowing whatsort of formations is responsible for his loss ofcirculation, it .follows that difficulty isexperienced in finding anyone material which will seal the pores and cavities of the variousformation with equal effectiveness.

As a prerequisite, it is obviously necessary that the material functionin a manner to establish .a seal on the inside of a porous formation ortheft structure. Also, the material must be tough and strong towithstand the increasingly greater pressures encountered in drilling,and the material must seal the various formations although withoutproducing a coating or sheath protruding into the well bore and commonlyknown as a filter cake, since otherwise the seal would be destroyed bythe drill in its movements up and down the well bore.

A basic feature ofthis invention is to provide a sealing compositionhaving a fiber mass to form a bridging mat within a porous, formationwith which a granular filler, whichmay be clay particles and/ or othergranular particle additives, may cooperate to form an impervious-seal.

The materials of the present invention adequately fulfill the aboverequirements for a successful all-purpose additive, since they areprimarily composed of tough and flexible fibers that have the ability towork their way within and establish a bridge or mat within-widelyvaryingeizes of pores and cavities of the thieving formations toefiiectivelyplugand seal them, and which, in producing:a-seal,:do notdevelop a filter cake of any appreciable size projecting into the wellbore.

A still further very important object-of the'invention is to provide anadditive in conformity with the preceding objects,'which can be-employedto prevent loss of circulation when a porous structure is encountered inan oil bearing formation, to thereby enable completion of the well bore;and which can be readily removed, as by an acid treatment, when it isdesired to bring in the well.

Rag fibers Asset forth in my prior copending application, Serial No.267,700, the fibrous material may satisfactorily consist of a masso'f'loose fibers, interwoven or interlocked groups of fibers, such aspieces of cloth or the like, or a mixture; of both. Since theinterwoven, fibers of cloth fragments are already intermingled andinterlocked with each-other, this material will in general facilitateand hasten the forming of a mat or skeletal bridge within a porousformation;

At the present time, fibers are used in lengths up to about four inches.This limit is observed only because fibers in excess of that length havebeen found to clog the valves of the ,mud circulation pump. Insofar asthe.

fibers performance in the well bore is concerned, ,itis believed thatthe longer fibers of lengths approaching and exceeding four inches areextremely beneficial and efiective in deep penetrations of theintersticial structure and function to effect successivere-establishmcnt of seals within the formation which may have beenruptured by hydrofraction.

It should be especially noted that only flexible,.long fibers of hightensile strength can perform this :lastmentioned, highly valuablefunction.

I have found that the use of rags in'various forms, either chopped, cut,groundor shredded,.as the fibrous constituent ofa drilling mud additive,improves the same in all respects and has the advantage of cheapness,availability of the necessary raw materials, and inexpensive processingoperations. templates the use of rags, textile waste, and sweepings fromcarpet factories. Under the category of rags, there is included thefollowing materials: rayon, silk, cotton, linen, nylon, wool, jute,flax, hemp, mohair, burlap, ramie and bemberg and synthetic fibers inaddition to rayon and nylon. Rayon and silk waste have been pmcessed forpurposesof the present invention in addition to wiping rags, misprints,remnants, mill ends, carpet :and. rug waste and textile wastes whichproduce remnants of linen, canvas, cotton or wool cloth, and similarmaterial such ;as may be purchased under the general category of rags. 7

in the processing of rags, they may be.chopped,.cut, shredded or ground.Chopped or cut rags are exactly what the name implies. The rags havebeen chopped or cutv in various sizes and shapes, with some "square,others triangular, and still others 'as strips, allincluding large andsmall pieces ranging in area from one fiftiethof asquare inch to onesquareineh or even larger. The

particular size and fiber composition of the rags 'in" chopped conditionwill, of course, be varied, depending on the type of thieving formationfor which the additive of the invention may be designed. Also, thechopping or cutting of the rags may be performed as a special stepin theprocessing, although it is possible to buy rags in this condition on themarket. For shredding or grinding the rags, they may be eflicaciouslypassed through a hammer mill, or through a rag cutter. Rags passedthrough the hammer mill once are referred to as first run rags. Theproduct may be described as partly a fibrous mass and partly a mixtureof large and small'pieces of chopped rags or cloth. The firstru'nthrough ahammer mill may not completely grindthe rags to a 'fibrousmass, and various sized chopped pieces might remain.

It may therefore be necessary to pass'the rags through the mill at leasta second time to secure a substantially one hundred percent fibrousmass. The same are known as second run rags.

Any other means or processes may be employed for;

flexible fibers which will readily conform to :the shapes of varioussized cavities in the thieving formation. Accordingly, this massof'fibrous material readilyenters the interstices of the formation underthe pressure of'the drilling fluid and functions to effectively plug andseal within the formation, preventingfurth'er loss of drilling fluid.

A series of tests has been conducted :to determine the efficacy of ragsalone as asealingagent or loss circulation 1 material. The tests wereconducted with 2000 cc. ,of a mixture containing the additive in aproportion Qf-sthree pounds per barrel of mud, with a maximum, testpressure available of 2000 p. s. i. and a filter pack :of gravelscreened'through a one-half inch, screen and retainedcn In particular,the invention cona one-quarter inch screen. The following tablestabulate the results of three such tests:

From the foregoing, it will be apparent that rag fibers constitute avery effective material for establishing a seal in a porous or thievingformation to prevent the loss of circulation therein, and deeplypenetrate into the interstices of the formation.

Another admix subjected to a series of tests and found satisfactory wasa combination of rags constituting the fibrous material and sawdust asthe granular material. Sawdust having a particle size of approximatelyNo. 4 was employed in this particular composition, although differentformations may require a sawdust of a different particle size, and forgeneral use, it will be desirable to use sawdust of a graded size,ranging from 0.006 to 0.200 inch in diameter or larger. In addition tosawdust, rice hulls, cotton seed hulls and ground corn cobs may be usedas the granular filler material. More specifically, the fibers of thefibrous materials may range in length from oneeighth inch to four inchesor more, depending on the ability of the pump of the drilling fluid tohandle the large fibers. The sawdust or other granular material used inthe additive may have a composition as follows:

Percent Retained on No. 4 screen Passing No. 4 but retained on No. 8 5Passing No. 8 but retained on No. 16 l0 Passing No. 16 but retained onNo. 30 40 Passing No. 30 but retained on No. 50 30 Passing No. 50 15 Inthis specification, the term sawdust is used with the connotationemployed in the lumber industry, including not only the granularmaterial produced by the saw but also chopped, ground or comminutedshavings and pieces of wood, however produced, and which are graded andsupplied in the industry under the same screening numbers and processesand under the general terms of sawdust.

The proportion of fibrous material to granular filler material in thecomposition may be widely varied and any proportion may be employed froma very high fiber content additive consisting of rags either in cut orchopped form or in fibrous form, as ground or shredded rags, or acombination of the two, to one containing a major portion or very highcontent of granular filler. A satisfactory composition may range fromone-third fibrous material and two-thirds granular filler toseventy-five percent fibrous material and twenty-five percent granularmaterial. In general, however, over fifty percent of granular materialreduces the effectiveness of the sealing action. A more specific exampleconsists of an additive containing two-thirds rags and one-third sawdustgraded as to particle size, as above set forth.

The fibrous material including the rags as herein described may be usedas an additive for any and all types of drilling fluids and drillingmuds. Also, the same may be used following the addition to the fluid ofother sealing materials and/or other treating materials. In the grindingor shredding of the rags for use in the present additive, a textilefiber is produced which, of course, can be duplicated by the use of thefiber in its natural state and before the same is processed into cloth,carpeting oi the like. Rags, with or without a granular filler, willusually produce a satisfactory sealing material for preventing loss ofcirculation in the drilling of a well. As stated, a mixture of first andsecond run rags may be used, or the additive may comprise all first runrags or all second run rags.

Textile fibers, in the natural state, may be substituted for the rags.As described in the above identified patent of Campbell and Twining, thetextile fibers may consist of the following: jute, hemp, flax, mohair,lechuguilla fibers, synthetic fibers, cotton, cotton linters, wool andwool shoddy. An additive consisting of rags only or of textile fibersonly is naturally more expensive than an additive having some granularfiller, such as sawdust in admixture therewith. All the textile fiberswill produce a good seal when used alone, with the exception of hemp.When hemp is used, a granular filler should be added to the same, or thehemp fiber can be mixed with other textile fibers or with rags. In fact,any of the textile fibers can be mixed to produce a satisfactory seal,and a seal may result which may be better than any one fiber willproduce when used by itself. Also, rags may be substituted for thetextile fibers without loss of any of the desirable characteristics thatmake the present additive outstanding as regards its merit. Forcementing a particularly troublesome formation of the cavernous type, itis entirely possible to use the present additive in combination withcementitious material.

The use of strands or rolled or twisted fibers, as a source of textilefibers should not be overlooked. For example, various types of reclaimedor salvaged fibers, such as the rolled or twisted strands and cordsfound in the carcasses of automobile tires from which the rubber hasbeen reclaimed, may constitute a satisfactory source of fibers.

In a cementing operation using Portland cement, for instance, the cementis mixed with water, sometimes bentonite being added. To the cement andwater mixture, the present sealing material is added and the same isthen forced down the drill stem, shoving ahead of it the drilling fluidremaining in the drill stem. Specified amounts of water, cement andsealing material mixture are pumped into the drill stem, ultimatelycausing the mixture to enter the cavities of the thieving formation.Following this operation, drilling mud or water is again pumped into thedrill stem, forcing the residue of the cement mixture from the same.Drilling operations are discontinued until the cement has had time toharden, whereupon drilling is again resumed.

It is also possible to use the present additive in conjunction with thepresent conventional practice of applying a gel plug as a sealer. A gelplug makes use of bentonite, a clay substance that has the property ofexpanding and swelling when water is mixed with it. In this operation,the bentonite is mixed with oil which does not cause it to expand, andthe mixture is pumped down the drill stem into the thieving formation.The present sealing material can be added to the mixture of bentoniteand oil prior to the pumping operation. When the mixture reaches thethieving formation, it comes in contact with the water which causes thebentonite to expand and to take a set, which action seals off the porousarea so that drilling operations can be resumed.

Wood fibers wood fibers, in their natural state, are filled with'lignin, a resinous material. While the latter streng hens the fibers,it unfortunately renders them stiff and brittle. Since a stiff andbrittle fiber will not easily penetrate a pore, it does not form aneffective sealing material. It is therefore necessary to remove thelignin, or delignify the wood, before its fibers are suitable for a lostcirculation material. In addition, it is also desirable to remove allresins and tars from the wood and to render the latter chemicallyneutral and inactive.

In some instances, waste from wooden products, already so treated as tobe a satisfactory source of fibers, can be used. By using the off-fallsfrom certain manufacturing operations of wood products, a very durablepliable and flexible wood fiber is produced. For example, in theproduction of battery separators, the wood is cooked 'for about sixteenhours .at 212 F. in a solution of .threepercent caustic soda. The Woodis then washed and rinsed to remove traces of the alakli. The treatmentalso removes all resins and tars from the wood. The'offfalls from such awood manufacturing operation or the chips, waste pieces and shavings ofany species of wood can be so treated and then ground or shredded in ahammer or attrition mill or other machinery to produce a mass of fibrouswood which is strong and flexible, and which will function verysatisfactorily as a drilling fluid additive. I

The wood may be reduced to a mass of fibers in any known manner. Forexample, grinding or shredding a block of wood, while wet, has beenfound to produce .suitable fibers. Practically every species of wood canbe used as a source of wood fibers. 'It is found to be generallypreferable to provide the wood in the form of thin slabs aboutone-quarter inch thick and to boil them in a three percent causticsolution for two to three hours to delignify them. Then, they are rinsedfor aboutten to thirty minutes in a boiling bath of a neutralizer torender them chemically neutral. Next, they are ground or shredded in anyknown manner to produce moist fibers.-

Thefibrous wood mass maybe supplemented to .increase its effectivenessas a drilling fluid additive by the addition thereto of a granularfiller, such as sawdust, rice hulls, cotton seed hulls or ground corncobs. To this compounded mixture may be added textile fibers or rags,either chopped, cut, ground o'r'sh'redded, in any desired proportion.jute, ramie, linen or other natural fibers or synthetic fibers and thelike may comprise the-sourcezfor the textile .fibers. By taking any ofthese fibers and adding sawdust, graded as to particle size, a drillingfluidadditive of .merit can be produced. The percentage of the variousingredients may Some fibrous material must be present in order to bridgethe pores and cavities of the porous formation and withstand pressure.The fibers interlace with eachotherand function as an initial seal. As aresult, the pores, and cavities are progressively reduced in size. Thefunction of the filler material, suchas. the sawdust, is to complementthe fibers in plugging the mat formed by the fibers. Obviously, theproportions of thefibrous materiahwhether textile fibers, syntheticfibers, ragfibers or wood fibers and the granular filler material may-be widely varied .as expediency dictates. s

As above set forth, textile and rag 'fibers are entirely Cotton, cottonlinters, W001, W001 shoddy,

V satisfactory for performing 8 V g all types of sealing operations forpreventing loss of circulation of drilling fluids. It will be readilyapparent, however, that the high rag fiber content of a compositionmayprove to be a heavy expense in-sealing operations whichrequire a largenumber of barrels of the composition, in view of the present relativelyhigh cost of rag fibers. I have discovered .that .in some instances, thecost of the composition maybe materially lessened by the substitution ofwood fibers for varying portions of the rag fibers, thereby substitutinga less expensivematerial for a more expensive one. While suchsubstitution probably results in a poductwhich will stand somewhat lesspressure than will a composition consisting entirely of rag'fibers withagranular filler, the loss in .Extent'of Penetration of Fibers into TestBed Rupturiug Test N0. Material Tested Pressure Delignified Wood Fibers750 34 inch.

Further tests .under the sameconditions as tests 1-4 above butusingvaryingamounts of textilefibers and/or granular materialsfinaddition to the wood fibers gave excellentv results, as follows:

Extent of Penetration of Fibers into Test Bed, inches Drilling FluidLost Before Rup- ' turing Pressure,

, lbs.

Pressure Sustained, lbs.

Test;

No. Material Tested 50% -Deligni-fied Wood Fibers and 50% N0. '43Sawdust.

% Delignified Wood Fibers and 10% Ground 'Rags.

45% Delignified .Wooi 2,000

Fibers, 45% No.

Sawdust and 10% V Chopped Bags.

50% Delignifi'e 'Wood Fibers, 25% .No. 4 Sawdust and 25% Ground'Rags.

50% Delignified Wood Fibers, 25% No. 4 Sawdust and 25% Chopped Bags.

' tional ,strengthis not required, the much cheaper wood fibersmaybesubstituted for the more costly textile *fibers. The 'cost saving isvery appreciable in view of the large quantities of sealing compoundwhich may be required. For example, the cost of treating a well torestore lost circulation by the use of a sealing compound as-a drillingfluid additive may frequently exceed $5,000.00. When cavernous orexcessively porous formations are encountered, it maybe necessaryto-completely fill:them by .bulk material before returns :can -berestored. For such purposes, the much lessexpensive wood fibersiareeconomically-preferable to the use of the, more expensive textile fibersalone.

" 7 Acid dissolvable fibers When aloss of circulation occurs in an oilproducing zone orregion, as when the drilling operation has reached andpenetrated into the oil pro'ducing sand, a different situation .exits'with regard to combatting lost circulation from those previouslydiscussed. In this latter situation, it isof course desired andnecessary to prevent a loss of circulation and restore or regainreturns, in order that the drilling operation may be completed as aprerequisite to completing or bringing in the well. However, it isessential at this point not to employ such a sealing material oradditive that the pore formation will be permanently sealed and thusprevent flow of oil into the well bore. Instead, it is desired toestablish a temporary seal which will prevent loss of returns until thedrilling operation is completed, but which may be readily removed byvarious conventional operations, as for example, the usual acidtreatment for completion of a well bore, to thus clear the pores of theformation of the sealing and obstructive efforts of the additive andpermit free flow of oil from the formation into the well bore.

To accomplish this purpose, the present invention employs under theseconditions a lost circulation material or additive which will establishan effective seal in the manner set forth hereinbefore, but which may bereadily removed by conventional treatments, such as acidizing.Accordingly, fibers are chosen as additives which may be readilydigested or dissolved by the acid treatment of completing a well. Rayonand nylon fibers being readily dissolvable in acid constitute a highlysatisfactory fiber for a sealing additive. Silk fibers and rags are alsoacceptable for this purpose. Wool fibers and rags can likewise beemployed although they are not as readily digested by the acid as thepreviously mentioned fibers.

In introducing a lost circulation additive for maintaining circulationin an oil producing zone, fibers and rags from the above set forthgroups are introduced into the well bore in the drilling fiuid in any ofthe conventional manners, and establish a sealing of the porousformations. After the well drilling operation is completed, and it isdesired to bring in the well, the lost circulation additive is removedfrom the porous formation by digesting the same with acids, causticsolutions or in any other manner.

The additive is thus effective to establish a highly effective seal toprevent lost circulation in the oil bearing formation; but can bereadily digested or removed as a prerequisite to or as a step in thebringing in of the well.

From the foregoing, the invention will be readily understood and furtherexplanation is believed to be unnecessary. However, since numerousmodifications and changes will readily occur to those skilled in theart, it is not desired to limit the invention to the exact embodimentsdisclosed, and accordingly, all suitable modifications and equivalentsmay be resorted to, falling within the scope of the appended claims.

What is claimed as new is as follows:

1. A process of producing a sealing additive composed of delignified,flexible and tough wood fibers for drilling fluids which comprisestreating wood, while in the form of bodies of unseparated fibers, saidbodies being of such size as to produce fibers having a length of from 4inch to about 4 inches, with an agent to remove lignin from the cellularstructure thereof, rinsing said bodies to remove the treating agenttherefrom, shredding the delignified wood bodies to thereby reduce thesame to a mass of loose fibers for use as an additive.

2. The process of claim 1 wherein the shredding operation is performedwhile the Wood bodies are wet.

3. The process of claim 1 wherein the shredding operation is performedwhile the wood bodies are wet from the rinsing operation.

4. The process of claim 1, wherein the bodies of wood are in the form ofthin slabs about inch in thickness.

5. The process of claim 1, wherein the mechanical separation is effectedby shredding the bodies of wood after the delignification of the fibersis complete and while the fibers are still wet.

References Cited in the file of this patent UNITED STATES PATENTS1,807,082 Boynton May 26, 1931 2,019,452 Hartford Oct. 29, 19352,061,616 Dreyfus Nov. 24, 1936 2,064,936 McQuiston Dec. 22, 19362,119,829 Parsons June 7, 1938 2,214,366 Freeland et al Sept. 10, 19402,398,347 Anderson Apr. 16, 1946 2,599,745 Campbell et al. June 10, 1952FOREIGN PATENTS 12,149/33 Australia Apr. 10, 1934 OTHER REFERENCESLangston: Fibrous Materials Aid Restoring Lost Drilling WellCirculation, Article in The Oil and Gas Journal, April 23, 1936, pages31, 33 and 34.

Silent: Circulation Losses, Article in The Oil and Gas Journal, May 21,1936, pages 72, 75, 78, 79 and 80.

Sawdon: Lost Circulation in Rotary Holes, Article in The PetroleumEngineer, February 1936, pages 27 to 30.

1. A PROCESS OF PRODUCTING A SEALING ADDITIVE COMPOSED OF DELIGNIFIED,FLEXIBLE AND TOUGH WOOD FIBERS FOR DRILLING DLUIDS WHICH COMPRISESTREATING WOOD, WHILE IN THE FORM OF BODIES OF UNSEPARATED FIBERS, SAIDBODIES BEING OF SUCH SIZE AS TO PRODUCE FIBERS HAVING A LENGTH OF FROM1/4 INCH TO ABOUT 4 INCHES, WITH AN AGENT TO REMOVE LIGNIN FROM THECELLULAR STRUCTURE THEREOF, RINSING SAID BODIES TO REMOVE THE TREATI-NGAGENT THEREFTOM, SHREDDING THE DELIGNIFIED WOOD BODIES TO THEREBY REDUCETHE SAME TO A MASS OF LOOSE FIBERS FOR USE AS AN ADDITIVE.